Starting systems for gas turbine engines



Feb. 13, 1962 K. H. GREENLY 3,020,716

STARTING SYSTEMS FOR GAS TURBINE ENGINES Filed Oct. '7, 1959 2 Sheets-Sheet 1 PUMP DELIVERY CONTROL ENGINE MECHANISM/ TORQUE \Nv am-oa Kenna-m H- GREG-ND! ATTORNEY! Feb. 13, 1962 K. H. GREENLY 3,020,716

STARTING SYSTEMS FOR GAS TURBINE ENGINES Filed Oct. 7. 1959 2 Sheets-Sheet 2 PRESSURE RELIEF VALVE TO SET SERVO PRESSURE fi sERvo PUMP - PRESSURE RELIEF VALVE 48 49 TO LIMIT SYSTEM NH-Ow PRESSURE DiSTRlBUTOR F 1 VALVE i 53 i PUMP CONTROL 5o PISTON 56 TO PUMP y L/" DUMP VALVE OUTFLOW 46 DISTRIBUTOR VALVE Y 46 PRESSURE SIGNAL FROM PUMP DELIVERY PRESSURISING XEE U LR SERVO SYSTEM H61 3 PRESSURE lNvEN-roR KENNETH H .GREENLY ATTORNEYS United States Patent 3,026,716 STARTING SYSTEMS FOR GAS TURBINE ENGINES Kenneth Howard Greenly, Luton, England, assignor to D. Napier & Son Limited, London, Engiand, a company of Great Britain Filed Oct. 7, 1959, Ser. No. 844,921 Claims priority, appiication Great Britain Aug. 8, 1957 2 Claims. (Cl. 60--39.14)

This invention relates to battery-operated starting systems for internal combustion engines, more particularly, but not exclusively, aircraft engines of the gas turbine t e.

iA conventional battery-operated starting system for such an engine comprises a direct current electric starter motor connected to a battery through series resistances which are cut out progressively during the starting operation as the speed of the engine increases. The over all efliciency of such a starting system is low because a substantial proportion of the energy supplied from the battery is dissipated in the series resistances, and so the battery has to be larger than it would need to be in a more efficient system. Since batteries are relatively heavy this is a serious disadvantage in aircraft where weight saving is of paramount importance.

It is an object of the present invention to provide a battery-operated starting system of greater overall efficiency, enabling a smaller battery to be employed. In one form of starting system embodying the invention, for a turbo-prop aircraft engine, the battery is about half the size required for the conventional starting system for this engine.

According to the present invention a battery-operated starting system for an internal combustion engine comprises 'a direct current electric motor supplied from the battery and driving a constant speed variable displacement hydraulic pump'which feeds a hydraulic motor which drives the engine.

Since the hydraulic pump is driven at a constant speed the electric motor can be directly connected to the battery without employing series resistances, so 'there is no avoidable waste of energy in the electrical part of the system.

The system is so arranged that during the first part of the acceleration of the engine the volumetric delivery of the hydraulic pump progressively increases while the delivery pressure remains constant so that the hydraulic motor provides constant torque. This applies up to the full power rating of the hydraulic pump, and during the remainder of the acceleration the pump operatm at constant hydraulic power, i.e. the pump delivery pressure falls as the engine speed continues to rise,

'In this manner the characteristics of the hydraulic motor conform relatively closely to the torque-speed requirement of the engine. In the case of a gas turbine engine, for best results the transition from constant hydraulic torque to constant hydraulic power should occur approximately at the light-up speed of the engine, be cause at lightrup there is an abrupt fall in the torque required to drive the engine. The torque required to drive the engine, which up to this point has been increasing with increasing speed, then begins to fall with increasing speed. When the pump is operating at constant hydraulic power, i.e. the falling delivery pressure, the torque available from the hydraulic motor falls with increasing engine speed.

Although at higher engine speeds theefficiency of the present system may be lower than the efficiency of the corresponding conventional electrical system, at low engine speeds where the major proportion of the total start ing energy is consumed the efiiciency of the present system is substantially higher than that of the corresponding conventional electrical system. Thus, the overall efiiciency of the present system is substantially higher than the overall efiiciency of the conventional electrical system.

The invention may be performed in various ways and one particular embodiment as applied to a twin engined turbo-prop aircraft will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 is a diagram of the starting system;

FIGURE 2 is a graph showing the torque-speed charactertistics of one engine and its starting motor during the starting period; and I FIGURE 3 is a diagram showing the control mechanism of the hydraulic pump.

Referring to FIGURE 1 of the drawings, a volt battery 10 is connected by terminals 11 to a charging system (not shown) which includes direct current electric generators driven from the two engines 12 and 13 during flight. For starting the engines from rest, the battery 10 is connected through a switch 14 to a direct current electric motor '15 which is mechanically coupled to a hydraulic pump 16. The pump 16 is of the variable displacement type, for instance a variable stroke reciprocating pump, the displacement and hence the volumetric delivery per revolution being adjusted by control mechanism 42 to be described in more detail hereinafter in connection with FIGURE 3. The engine '12 is provided with a hydraulic starting motor 17 which is connected to the engine through an over-running clutch 18. The engine 13 is similarly provided with a starting motor 19 connected to it by an over-running clutch 20.

The hydraulic pump 16 receives hydraulic fluid from a reservoir 21 through a pipe 22. The delivery side of the pump 16 is connected to a pipe 23 leading to a T-junction 26 from which individual branches 27 and 28 lead to the starting motors 17 and 19 respectively. A starting valve 29 is provided in the branch 27 and a starting valve 30 is provided in the branch 28,. Hydraulic fluid returns from the hydraulic motors 17 and 19 through piping-31 to the reservoir 21.

Referring now to the control mechanism for the hydraulic pump, which is illustrated in FIGURE 3, a servo pump 43 provided with a by-pass pressure relief valve 44 delivers a supply of fluid at constant pressure into a control valve assembly 45. Outflow from the valve assembly 45 through a line 46 is controlled by a pressurising valve 47. In the valve assembly 45 the flow divides into two branches 48 and 49. Connected between the branches 48 and 49 is a pump control servo-cylinder 50 containing a piston 51 which is connected to the pump 16 by a link 52 (see also FIGURE 1). The position of the piston 51 in the cylinder 50 determines the volumetric delivery of the pump per revolution. The piston 51 is urged towards the right in 'FIGURE 3 by a spring 61,an d its position is determined by the pressure'difference between its left-hand and right-hand faces. This pressure difference is in turn determined by the position in relation "to the two branches 48 and '49 of an inflow distributor valve 53, an outflow distributor valve 54 and a dump valve 55,. These three valves are all connected to a common yoke 5d the position of which is determined by a bellows 57 loaded by a spring 58. The interior of the bellows is in communication with the delivery side of the pump 16 through a p e e s na i e 9 ee l o F GU E nd w h a pressure relief valve (50.

The operation of the control mechanism shown in FIG- URE 3 is as follows. Assuming that the pump 16 is being driven by the electric motor 15, it will create a certain delivery pressure which, if neither of the starting valves 29 or 30 is open, will build up to a maximum which is determined by the pressure at which a relief valve 60 opens. As the pressure builds up, the bellows 57 expands moving the inflow and outflow distributor valves 53 and 54 and the dump valve 55 from the positions shown in full lines in FIGURE 3 toward the left-hand end positions shown in dot and dash lines, so that the valves 53 and 54 partially close the left-hand branch 48 while the portion of the dump valve 55 in this branch opens relieving the pressure in this branch between the valves 53 and 54.

At the same time the portion of the dump valve in the right-hand branch 49 closes and the valves 53 and 54 move out of this branch so that the full pressure of the pump 43 is applied to the right-hand side of the piston. 51. The piston 51, therefore, moves toward the left from the position shown in full lines toward the position shown in dash-dot lines in FIGURE 3 and reduces the delivery of the pump 16. When the maximum pressure is attained the bellows 57 will be fully expanded, the inflow and outflow distributor valves 53 and 54, and the dump valve 55, will be in their left-hand end positions so that the servo piston 51 is forced into its left-hand end position at which the pump delivery is virtually zero. As one of the starting valves is opened, permitting fluid to flow to the corresponding hydraulic starting motor, the pump delivery pressure will be partially relieved so that the bellows will begin to contract. This will cause the valves 53, 54 and 55 to move a corresponding distance to the right to the positions shown in dash-dot lines in FIGURE 3, so that the valves 53 and 54 partially close the righthand branch 49 while the portion of the dump valve 55 in this branch opens, relieving the pressure in this branch between the valves 53 and 54, causing the servo piston 51 also to move towards the right to increase the volumetric delivery and restore the pressure to its previous value. Thus up to the full capacity of the pump, its volumetric delivery per revolution is automatically adjusted to maintain a substantially constant delivery pressure.

The starting cycles for the two engines 12 and 13 are identical, so it will be sufiicient to describe that for the engine 12 only. When the switch 14 is closed the electric motor 15 is energised and drives the hydraulic pump 16. The pump will deliver at the maximum pressure determined by the mechanism 42 as described above, and will thereby provide a constant load on the motor 15 independently of the volumetric delivery per revolution of the pump. Consequently the motor will, under these circumstances, run at a constant speed. When the starting valve 29 is opened, the hydraulic motor 17 will receive hydraulic fluid at the said pressure. Up to the limit of the pump capacity the said circumstances will continue to apply, the pressure of the hydraulic fluid at the inlet side of the motor 17 remaining constant, and the quantity of fluid supplied being automatically adjusted by the mechanism 42 to maintain this pressure. Thus the motor 17 will produce a constant torque as indicated by the horizontal line 33 in FIGURE 2. This torque is adequate to rotate and accelerate the engine 12. As the engine 12 begins to rotate and accelerate, the volumetric delivery of the pump 16 per revolution is progressively increased, without change in the speed of the pump since the load on the electric motor 15 remains constant, to supply an increasing quantity of hydraulic fluid at the said constant pressure to the hydraulic motor 17. This condition continues until the pump 16 is operating at its full capacity, i.e. at its maximum volumetric delivery per revolution, as designated by the point 34 in FIGURE 2. In FIGURE 2 the full line curve represents the torque-speed characteristic of the motor 17 and the dotted line curve represents the negative torque-speed characteristic of the engine 12, i.e. it indicates the torque required to turn and rapidly accelerate the engine at any given speed in the starting range. The capacity or maximum volumetric delivery per revolution of the pump 16 is selected so that at the speed of the motor 17 corresponding to the point 34 the engine 12 is being driven fast enough to enable light-up to occur. The increasing torque required to accelerate the engine 12 rapidly up to light-up speed is indicated by the portion 35 of the dotted line curve in FIGURE 2. When light-up occurs, at the point 36, the torque required to continue accelerating the engine is reduced to the value represented by the point 37. Thereafter the torque requirement progressively decreases in accordance with the curve portion 38 as the engine speed rises, until the engine speed has passed the point 39 at which it is self-sustaining. When the speed corresponding to the point 39 has been exceeded the starting system can be shut down. At speeds beyond that corresponding to the point 34, ie the maximum volumetric delivery per revolution of the pump 16, the pump can no longer supply sufiicient fluid at the said constant pressure to the hydraulic motor 17, and consequently the hydraulic pressure falls. As the pressure falls, the load on the electric motor 15 also falls, permitting it to accelerate and drive the pump 16 faster, the speed automatically relating itself to the pressure so that the power consumed by the pump 16 remains constant. The pump 16 thus automatically delivers the increasing quantity of fluid per unit time required by the hydraulic motor 17, but at a progressively falling pressure. The torque-speed characteristic of the motor 17 therefore falls, as represented by the line 46 inFIGURE 2. The falling torquespeed characteristic of the hydraulic motor 17 above the light-up speed as represented by the line 40 is permissible, since above light-up speed the torque-speed requirements of the engine 12 as represented by the line 38 also falls. At the point 41, which is just past the self-sustaining speed of the engine, the starting system is shut down by closing the valve 29 and reducing the pump delivery to zero.

The engine 13 can then be started in an exactly similar manner.

This is a continuation-in-part of my application Serial No. 753,801, filed August 7, 1958, now abandoned.

What I claim as my invention and desire to secure by Letters Patent is:

l. A system for supplying hydraulic fluid to a hydraulic starter motor of a gas turbine engine having a predetermined light-up speed, comprising an electric battery, a direct current electric motor having a falling load-speed characteristic, electrical connections from said battery to said electric motor, a variable displacement hydraulic pump, fluid connection means connecting said pump to said hydraulic starter motor, a driving connection between said electric motor and said pump, means for adjusting the volumetric delivery per revolution of said pump, means for sensing the delivery pressure of said pump, and an operative connection between said sensing means and said adjusting means, said sensing means, said operative connection and said adjusting means together serving to produce a reduction in volumetric delivery per revolution upon a rise in delivery pressure and conversely an increase in volumetric delivery per revolution, up to the maximum volumetric delivery per revolution, upon a fall in the delivery pressure, whereby said delivery pressure and the load imposed by said pump on said electric motor and the speed of said electric motor remain substantially constant up to said maximum volumetric delivery per revolution of said pump, and whereby a fall in delivery pressure at said maximum delivery per revolution of said pump reduces the load imposed by said pump on said electric motor and thereby increases the speed of said electric motor and said pump, and in which the maximum volume-ric delivery per revolution of said pump and the volumetric capacity per revolution of said hydraulic starting motor are so selected that said maximum volumetric delivery of said pump is attained approximately at said light-up speed.

2. A hydraulic fluid supply system according to claim 1 in which said means for adjusting the volumetric delivery per revolution of said pump comprises a source of hy- 6 draulic pressure, a hydraulic servo motor having a piston, mines the pressures acting upon opposite sides of said conduit means connecting said source of hydraulic prespiston. sure to opposite sides of 1said giston, the plajosition1 of said References Cited in the file of this patent piston depending upon t e di erence in ydrau ic pressures acting upon opposite sides thereof said means for 5 UNITED STATES PATENTS sensing the delivery pressure of said pump comprises a 2,161,439 Thoma June 6, 1939 bellows the extension of which corresponds to said delivery 2,382,437 Molly Aug. 14, 1945 pressure, and said operative connection between said sens- 2,562,615 Huber July 31, 1951 ing means and said adjusting means comprises valve means 2,628,476 Grier Feb. 17, 1953 in said conduit means the position of which valve means 10 2,711,071 Frankel June 21, 1955 is determined by the extension of said bellows and deter- 2,741,989 Postel et a1. Apr. 17, 1956 

